Thin-film photovoltaic power element with integrated low-profile high-efficiency dc-dc converter

ABSTRACT

A photovoltaic device includes at least one photovoltaic cell and a DC/DC converter electrically coupled to the at least one photovoltaic cell. The at least one photovoltaic cell and the DC/DC converter are integrated into a photovoltaic package.

BACKGROUND

The present invention is directed generally to photovoltaic systems andmore specifically to a photovoltaic element with an integrated DC/DCconverter.

Development of new technologies for low-cost manufacturing of thin-filmphotovoltaic (PV) power cells is enabling new types of buildingmaterials that integrate photovoltaic power generating elements. In thisrole, the photovoltaic modules become architectural elements, requiringproperties such as a low profile, ease of connection to the utilitysystem, and the ability to maximize energy capture in a complex physicalenvironment having shadows, reflections, and differing orientations.

An example is the residential roof shingle, where it is desired that thephotovoltaic modules have the appearance of asphalt shingles. Tomaximize energy capture on a complex multifaceted roof, smartcontrollers are required that can track PV peak power points on a finescale.

SUMMARY

One embodiment relates a photovoltaic device. The photovoltaic deviceincludes at least one photovoltaic cell and a DC/DC converter. The DC/DCconverter can be electrically coupled to the at least one photovoltaiccell. The at least one photovoltaic cell and the DC/DC converter areintegrated into a photovoltaic package.

Another embodiment relates to a method of operating a photovoltaicmodule. A photovoltaic cell power output is provided to a DC/DCconverter having at least two transistors. The at least two transistorsare switched only when a string current is not in a nominal range. Avoltage is provided to a string connection of the DC/DC converter.

Another embodiment relates to a photovoltaic converter circuit. Thecircuit includes at least one photovoltaic cell and a DC/DC converter.The DC/DC converter includes a buck converter, a boost converter, and apass-through. The buck converter includes at least one first transistor.The boost converter includes at least one second transistor. The buckconverter is electrically coupled to the boost converter. The DC/DCconverter is electrically coupled to the at least one photovoltaic cell.

Another embodiment relates to a photovoltaic system. The photovoltaicsystem includes an inverter and at least two photovoltaic modules. Eachof the at least two photovoltaic modules include at least onephotovoltaic cell and a DC/DC converter. The DC/DC converter iselectrically coupled to the at least one photovoltaic cell. The DC/DCconverter includes a pass-through. The DC/DC converters of the at leasttwo photovoltaic modules are electrically connected in series with theinverter.

Another embodiment relates to a method of operating a photovoltaicsystem. A plurality of photovoltaic modules in series provide a poweroutput. At least one of the plurality of photovoltaic modules operatesin a pass-through mode. At least one of the plurality of photovoltaicmodules operates in a boost mode. At least one of the plurality ofphotovoltaic modules operates in a buck mode. A substantially constantstring voltage is provided to an inverter. At other times, optionally,the plurality of photovoltaic modules all operate in pass-through mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a series string of photovoltaic modules connectedto a central inverter in accordance with a prior art embodiment.

FIG. 2 is a graph of a current-voltage curve of a typical photovoltaiccell in accordance with a prior art embodiment.

FIG. 3 is a diagram of a series string of photovoltaic modules withlocal DC/DC converters connected to a central inverter in accordancewith a prior art embodiment.

FIG. 4 is a diagram of a series string of photovoltaic modules withlocal bidirectional DC/DC converters connected to a central inverter inaccordance with a prior art embodiment.

FIG. 5 is a diagram of a series string of photovoltaic element moduleswith integrated DC/DC converters connected to a central inverter inaccordance with a representative embodiment.

FIG. 6 is a diagram of the photovoltaic element module of FIG. 5 inaccordance with a representative embodiment.

FIG. 7A is a side view of a first photovoltaic package of FIG. 6 inaccordance with a representative embodiment.

FIG. 7B is a side view of a second photovoltaic package of FIG. 6 inaccordance with a representative embodiment.

FIG. 7C is a side view of a third photovoltaic package in accordance ofFIG. 6 with a representative embodiment.

FIG. 7D is a side view of a fourth photovoltaic package in accordance ofFIG. 6 with a representative embodiment.

FIG. 8 is a circuit of the photovoltaic element module of FIG. 6 inaccordance with a representative embodiment.

FIG. 9 is a circuit of a controller of the photovoltaic element moduleof FIG. 8 in accordance with a representative embodiment.

FIG. 10 is a graph of a constant power source characteristic of anautonomous photovoltaic element module in accordance with arepresentative embodiment.

FIG. 11 is a graph of a predicted efficiency of DC/DC converter powerstages under various operating conditions in accordance with arepresentative embodiment.

FIG. 12 is a graph of a simulation of turn-on transients of a tenphotovoltaic element module system in accordance with a representativeembodiment.

DETAILED DESCRIPTION

A device, method, and circuit of a photovoltaic power element module aredescribed. In the following description, for purposes of explanation,numerous specific details are set forth to provide a thoroughunderstanding of exemplary embodiments of the invention. It will beevident, however, to one skilled in the art that the invention may bepracticed without these specific details. The drawings are not to scale.In other instances, well-known structures and devices are shown insimplified form to facilitate description of the representativeembodiments.

Since thin-film PV cells can be very thin and lightweight, it isdesirable that smart controllers be thin and lightweight as well.Integration of a smart controller directly into a photovoltaic panel ischallenging because of the high ambient temperatures encountered, aswell as the very low profile required. Hence, a new system configurationand power converter approach that can operate with high efficiency whilemeeting size (low profile) requirements is desired. A thin-filmphotovoltaic module for building-integrated applications, having locallow-profile dc-dc converters integrated into a package such as alaminate, providing maximum power point tracking on a fine scale,interfacing to a dc output, and operating with high efficiency isdescribed.

Photovoltaic cells produce direct current (DC) voltage of a fraction ofa volt, while the utility system wiring within buildings typicallyemploys alternating current (AC) voltages greater than 100 V. Thus, thevoltage must be increased and changed to AC form. Referring to FIG. 1, adiagram of a series string of photovoltaic modules connected to acentral inverter in accordance with a prior art embodiment is shown.Photovoltaic modules 110 are connected in series with an input of aninverter 120. An output of the inverter 120 is connected to an ACutility 130. The photovoltaic modules 110 can include multiplephotovoltaic cells connected in series. Optionally, the photovoltaiccells include backplane or bypass diodes. The photovoltaic modules 110produce a low-voltage DC output of typically several tens of volts. Thephotovoltaic modules 110 are connected in series, to achieve ahigh-voltage DC V_(string) that is connected to the input terminals ofthe inverter 120. The inverter 120 produces AC as required to interfaceto the AC utility 130.

Referring to FIG. 2, a graph of a current-voltage curve of a typicalphotovoltaic cell in accordance with a prior art embodiment is shown.When illuminated, photovoltaic cells exhibit a current-voltage (i-v)characteristic 210. The power generated by the cell is maximized at acertain voltage and current known as a maximum power point 220. Thecurrent and voltage at the maximum power point 220 vary with solarirradiance, as well as with other factors such as orientation, aging,temperature, etc. To maximize energy capture, it is desirable to operateevery cell at its maximum power point 220.

Referring again to FIG. 1, when photovoltaic modules 110 are connectedin a series string, they operate with the same current I_(string). Ifall photovoltaic modules 110 are identical, then it is possible for amaximum power point tracking controller within the inverter 120 toselect the current I_(string) such that each of the photovoltaic modules110 operates at its maximum power point. However, when factors such asshadows, shading, reflections, temperature differences, and differingorientations cause the current-voltage characteristics of thephotovoltaic modules 110 to vary, then the cell maximum power points canoccur at different currents, and there is no single choice of I_(string)that causes each of the photovoltaic modules 110 of the series-connectedstring to produce its maximum possible power.

A typical example is the partial shading of a series-connected string ofphotovoltaic cells. In this case, the shaded cells are not capable ofproducing as much current as the fully illuminated cells. The maximumpower points of cells in the series string occur at different currents,and there is no single current i=I_(string) that causes every cell togenerate its maximum power. Smaller scale control is desired that canoperate smaller blocks of photovoltaic cells at or nearer their maximumpower points, where the smaller blocks have an optimal current differentthan the series string current I_(string).

Referring to FIG. 3, a diagram of a series string of photovoltaicmodules 110 with local DC/DC converters 340 connected to a centralinverter 120 in accordance with a prior art embodiment is shown. Thephotovoltaic modules 110 are each connected to a DC/DC converter 340.The DC/DC converters 340 are connected in series with an input of aninverter 120. An output of the inverter 120 is connected to an ACutility 130. The DC/DC converters 340 allow the current of theindividual photovoltaic modules 110 to differ from the string current.

The DC/DC converters 340 can be buck, boost, and single-switchbuck-boost converters or a combination thereof. However, thesingle-switch buck-boost type converters have lower efficiencies. Asused herein, “buck” and “boost” converters mean any converter thatdecrease and increase the voltage respectively, and include buckconverter circuits, boost converter circuits, SEPIC converter circuits,and Cuk converter circuits. The DC/DC converters 340 are designed tooperate with a range of duty cycles, for example, approximately1.5:1-3:1, with the nominal operating point occurring near the middle ofthe range. The total voltage produced by the string of DC/DC converters340 can vary widely with operating point. The total string voltage ofthe DC/DC converters 340 is fed into the inverter 120 for interface tothe AC utility 130. However, the series string of photovoltaic modules110 with local DC/DC converters 340 connected to a central inverter 130has losses and relatively low efficiency. The added losses incurred atfull power may offset the added energy captured under partially shadedconditions, negating any advantages.

In another prior art embodiment, a “shuffle” approach is employed.Referring to FIG. 4, a diagram of a series string of photovoltaicmodules with local bidirectional DC/DC converters connected to a centralinverter in accordance with a representative embodiment is shown. Thephotovoltaic modules 110 are each connected to a bidirectional DC/DCconverter 445. The bidirectional DC/DC converters 445 are connected inseries with an input of an inverter 120. An output of the inverter 120is connected to an AC utility 130. If all photovoltaic modules areidentical, then no current flows through the bidirectional DC/DCconverters 445 (i.e. the “shuffle” converters); this leads to desirablehigh efficiency at full power.

A partially shaded photovoltaic module 110 will require less currentthan I_(string). The excess current will flow through the “shuffle”converters. An isolated and bidirectional DC/DC converter 445 converteris connected between the top and bottom photovoltaic modules 110. Thisapproach is effective in addressing the problem of efficiency at fullpower, but requires more complex interconnections and bidirectionalDC/DC converters 445. It also has the disadvantage of requiring anisolated and bidirectional DC/DC converter 445 to terminate the string.

In one representative embodiment, the power transistors of an integratedDC/DC converter switch only when needed. The photovoltaic element moduleprovides a new type of photovoltaic dc power module, to meet the needsof building-integrated photovoltaic systems. The photovoltaic elementmodule includes an array of series-connected thin-film photovoltaiccells, an in-package, low-profile, high-efficiency DC/DC converter, andan in-package controller. The DC/DC converter and its controller allowmaximum power point tracking on a fine scale, and interfacing thephotovoltaic element modules to a series string. The photovoltaicelement modules employ a dc-dc converter that switches the powertransistors (which incurs power loss) only when needed—i.e., only whenthere are variations in module i-v characteristics.

When all photovoltaic element modules have identical i-vcharacteristics, the DC/DC converters connect their respective modulesdirectly to the string. Switching of the DC/DC converter powertransistors is employed only when needed to change the voltage orcurrent magnitudes in response to variations in photovoltaic module i-vcharacteristics. Thus, the photovoltaic element module improves energycapture in the complex physical environments that may be encountered inbuilding-integrated photovoltaic systems.

Referring to FIG. 5, a diagram of a series string of photovoltaicelement modules with integrated DC/DC converters connected to a centralinverter in accordance with a representative embodiment is shown. Thephotovoltaic element modules 505 are each connected in series with aninput of an inverter 120. An output of the inverter 120 is connected toan AC utility 130. The photovoltaic element modules 505 each include aphotovoltaic array 510, a buck converter 550, and a boost converter 560.The photovoltaic module 510 is connected to the buck converter 550; andthe buck converter 550 is connected to the boost converter 560. Theoutput of the boost converter 560 is connected to the output of thephotovoltaic element module 505. The photovoltaic array 510 can includemultiple photovoltaic cells connected in series. Optionally, thephotovoltaic cells include backplane or bypass diodes. The photovoltaicarray 510 produces a low-voltage DC output of typically several tens ofvolts.

It is desired to operate the string of photovoltaic element modules 505with an approximately constant total output voltage, allowing betteroptimization of the central inverter 120. This can be achieved throughDC/DC converters capable of both buck (voltage step-down) and boost(voltage step-up) operation. Under balanced conditions, the converterconnects the photovoltaic array 510 of the photovoltaic element module505 directly to the string. When a module is partially shaded, themodule's current will be less than the string current. Its DC/DCconverter operates in buck mode. Buck mode buffers the lower currentphotovoltaic module such that the converter output current equals thehigher string current. Buck mode also results in a lower contribution ofvoltage to the total string voltage. When a module is fully illuminatedbut other modules in the same string are shaded, then its DC/DCconverter operates in boost mode. In boost mode, the converter helps tobuffer the higher current photovoltaic module such that the converteroutput current equals the lower string current. Boost mode also resultsin a higher contribution of voltage to the total string voltage. Withthis mix of buck and boost operation amidst DC/DC converters in a stringof photovoltaic element modules 505, it is possible for the string totaloutput voltage to remain constant. This behavior can be achieved usingautonomous local module controllers that adjust their duty cycles toproduce a constant power characteristic at the converter output. Thecentral inverter may then adjust its average (DC) input current to thevalue that results in the desired total string voltage.

It is further desired that the maximum power point tracking algorithmsof the individual photovoltaic element modules 505 be independent andnon-interacting. One way to achieve this is by addition of a feedbackloop that regulates the voltage or current of the photovoltaic array tofollow a reference provided by a maximum power point tracker. Thisfeedback loop produces the changes in converter duty cycle necessitatedby changes in other modules of the series string, freeing the maximumpower point tracker of this function. This significantly improves thesystem dynamic performance.

Traditionally, inverters or converters are typically in enclosures thatare physically removed from the photovoltaic panels, withinterconnecting wiring. Prior art references to “integrated” convertermodules refer to mounting of a converter box on the back of thephotovoltaic module to eliminate the interconnecting wiring. Theembodiments of the present invention lead to a new level of integration,in which a very low profile DC/DC converter is constructed directly inthe package of a thin-film photovoltaic module. This enables newarchitectural building materials for integration of smart systems ofphotovoltaic power sources into buildings.

Referring to FIG. 6, a diagram of the photovoltaic element module 505 ofFIG. 5 in accordance with a representative embodiment is shown. Thephotovoltaic element module 505 is integrated into a photovoltaicpackage 607. The photovoltaic element module 505 includes a photovoltaicarray 510, an energy storage device, such as a capacitor 615 or anotherstorage device, a DC/DC converter 670, and a controller 680. The DC/DCconverter 670 can include a buck converter 550 and a boost converter 560or a buck-boost converter. The controller 680 controls the DC/DCconverter 670 using, for example, input from the a photovoltaic array510 and/or the capacitor 615. Optionally, the DC/DC converter 670 caninclude an electromagnetic interference (EMI) filter (not shown). Aplurality of photovoltaic element modules 505 can be electricallyconnected in series to an inverter (not shown) that typically includesfiltering. The inverter can be connected to an AC utility (not shown)including transient protection (not shown). Preferably, the photovoltaicelement module 505 does not include an EMI filter or an inverter.

The term package includes devices, such as the photovoltaic cells andcircuit elements, such as converters, enclosed between a front barrierand a back barrier. The front barrier is transparent to solar radiation.The front barrier may comprise glass, plastic and/or encapsulant. Theback barrier may comprise one or more glass, plastic and/or metal layersin a laminate or a plastic molded back piece. Examples of a packageinclude devices laminated between sheets of plastic or polymer material,such as polyethylene terephthalate (PET) and/or ethylene vinyl acetate(EVA) sheets; devices attached to a substrate, where at least some ofthe devices may be encapsulated in epoxy; and devices sealed between asheet of glass and a substrate (such as a glass or molded plasticsubstrate) and/or a sheet of plastic. In a monolithic integration of apackage, a single substrate can have multiple cells formed on it. Thissubstrate may or may not be used as part of the structure of the modulepackage. Alternatively, the substrate is omitted and the cells “float”in the encapsulant between the front and back barriers. The encapsulantfills the spaces between the devices and the barrier layers.Alternatively, the space(s) between the barrier layers is filled withair or gas, as in a double paned window.

A package can include multiple layers of different materials. A lowprofile package preferably has a height less than or equal to 11 mm suchas 3 mm-11 mm; for example, 3 mm-6 mm; specifically, 5 mm-6 mm.

Referring to FIG. 7A-D, side views of various photovoltaic packages ofFIG. 6 in accordance with a representative embodiment are shown. Thephotovoltaic package 607 may comprise a low-profile photovoltaiclaminate or non-laminate package. A laminate comprises multiple layersof materials formed together, such as cells 510 and DC/DC converter 670on the substrate encapsulated between two polymer or plastic sheets, asshown in FIG. 7D. The low-profile photovoltaic laminate has awidth-to-thickness ratio of about 30:1 to about 607:1 at its smallestwidth and the height is less than or equal to 11 mm. In otherembodiments, the thickness is less than 11 mm; such as 3 mm-11 mm; forexample, 3 mm-6 mm; specifically, 5 mm-6 mm. In a representativeembodiment, the photovoltaic package 607 is about the size of a typicalthree-tab residential roofing shingle. Alternatively, the photovoltaicpackage 607 can be a long sheet such as a roll of photovoltaic roofingmaterial laminated on both sides. The roll of laminated photovoltaicmodule material can be cut to length. The photovoltaic package 607 canbe any low-profile form and in any shape. Alternatively, thephotovoltaic package 607 can be a non-laminate type package such as aglass sheet covered package where the electrical components areencapsulated in a polymer encapsulant.

The photovoltaic package 607 comprises a device layer 791 and front andback barrier or encapsulation layers 794 and 790. In a representativeembodiment, the substrate (not shown for clarity) of each photovoltaicarray 510 is a sheet of metal such as aluminum or galvanized stainlesssteel; other plastic or glass materials may also be used. The substratecan be rigid or flexible. Photovoltaic arrays 510 can be attached to thesubstrate using an adhesive such as epoxy Alternatively, thephotovoltaic arrays 510 can be formed or printed directly on thesubstrate such as by sputtering methods shown in U.S. patent applicationSer. No. 10/973,714, titled Manufacturing Apparatus And Method ForLarge-Scale Production Of Thin-Film Solar Cells, filed Oct. 25, 2004 andU.S. patent application Ser. No. 11/451,616, titled Photovoltaic ModuleWith Integrated Current Collection And Interconnection, filed Jun. 13,2006 which are herein included by reference. The photovoltaic arrays 510are connected to each other by electrical connections 793. A capacitor615 and the DC/DC converter 670 can be integrated onto a separatesubstrate, such as a printed circuit board 792, which can then beelectrically attached to the photovoltaic arrays 510. Alternatively, theprinted circuit board 792 can be a flex circuit. Alternatively, thecapacitor and the converter can be attached, formed or depositeddirectly onto the encapsulation layers 794 and 790. The photovoltaicarrays 510 are connected to the printed circuit board 792 by electricalconnection(s) 793. The encapsulation layer 794 is formed over thephotovoltaic arrays 510 and the printed circuit board 792. The frontbarrier or encapsulation layer 794 can be a polymer layer, a sheet ofglass that is sealed to a sheet of polymer or plastic material such asPET or EVA that is bonded or laminated to the photovoltaic arrays 510and the other components, such as DC/DC converter 670. The back barrierlayer 790 is formed under the photovoltaic arrays 510 and the printedcircuit board 792, as described with regard to layer 794.

The electrical components such as capacitor 615 and DC/DC converter 670can be surface mounted to the printed circuit board 792 or incorporatedinto the printed circuit board 792. The electrical and other componentscan be encapsulated in epoxy and/or encapsulated by the encapsulationlayer 794. The printed circuit board 792 can have varying degrees ofintegration. For example, components such as the capacitors andinductors can be discrete components that are attached to the printedcircuit board 792. The main energy storage capacitor 615 can be aceramic capacitor attached to the printed circuit board 792.Alternatively, the main energy storage capacitor 615 can also be formedinto or onto the printed circuit board 792 itself. The various inductorsthat are part of the converter can be discrete components.Alternatively, the inductors can also be formed into or onto the printedcircuit board 792 itself. For instance, in a multi-level printed circuitboard, various trace patterns combined with vias, bond wires, or jumpwires can be used to fashion inductors. Alternatively, the printedcircuit board 792 can be made of flexible materials and consist ofmultiple and/or localized layers.

Referring to FIG. 7A, a side view of a first photovoltaic package ofFIG. 6 in accordance with a representative embodiment is shown. In thisembodiment, the front barrier layer 794 comprises an encapsulant and therear barrier 790 comprises a molded plastic substrate which supports thecells 520 and the circuit board 792. Referring to FIG. 7B, a side viewof a second photovoltaic package of FIG. 6 in accordance with arepresentative embodiment is shown. The illustrated photovoltaic package607 comprises a back barrier 714, a device layer 791, and a frontbarrier 711. In a representative embodiment, the back barrier 714 is asheet of metal such as aluminum or galvanized stainless steel; otherplastic or glass materials may also be used. The back barrier 714 can berigid or flexible. Photovoltaic arrays 510 can be attached to the backbarrier 714 using an adhesive such as epoxy Alternatively, thephotovoltaic arrays 510 can be formed or printed directly on the backbarrier 714 as described above. The photovoltaic arrays 510 areconnected to each other by electrical connections 793. A capacitor 615and the DC/DC converter 670 can be integrated onto a separate substrate,such as a printed circuit board 792, which can then attached to the backbarrier 714. Alternatively, the printed circuit board 792 can be a flexcircuit. Alternatively, the capacitor and the converter can be attached,formed or deposited directly onto the back barrier 714. The photovoltaicarrays 510 are connected to the printed circuit board 792 by electricalconnections 793. The front barrier 711 is located over the photovoltaicarrays 510, and the printed circuit board 792. The front barrier 711 canbe a sheet of glass. The front barrier 711 is sealed to the back barrier714 by an edge seal 712. The space between the front barrier 711, theback barrier 714, and the edge seal 712 is filled with an encapsulant713. Alternatively, the space can be filled with air or a gas such asargon.

Referring to FIG. 7C, a side view of a third photovoltaic package ofFIG. 6 in accordance with a representative embodiment is shown. Theillustrated photovoltaic package 607 comprises single glass laminate.The photovoltaic package 607 comprises a back barrier 714, a devicelayer 791, and a front barrier 711. In a representative embodiment, thefront barrier 711 can be a sheet of glass. Photovoltaic arrays 510 canbe attached to the front barrier 711 using an adhesive such as epoxyAlternatively, the photovoltaic arrays 510 can be formed or printeddirectly on the front barrier 711 as described above. The photovoltaicarrays 510 are connected to each other by electrical connections 793. Acapacitor 615 and the DC/DC converter 670 can be integrated onto aseparate substrate, such as a printed circuit board 792, which can thenattached to the front or back barrier. Alternatively, the printedcircuit board 792 can be a flex circuit. Alternatively, the capacitorand the converter can be attached, formed or deposited directly onto thefront barrier 711. The photovoltaic arrays 510 are connected to theprinted circuit board 792 by electrical connections 793. The backbarrier 714 is sealed against the edges of the front barrier 711. Theback barrier 714 is a sheet of plastic, or plastic and metal such asaluminum. The back barrier 714 can be rigid or flexible. The spacebetween the front barrier 711, the back barrier 714, and the edge seal712 is filled with an encapsulant 713. Alternatively, the space can befilled with air or a gas such as argon.

Referring to FIG. 7D, a side view of a fourth photovoltaic package ofFIG. 6 in accordance with a representative embodiment is shown. Theillustrated photovoltaic package 607 comprises a flexible laminate. Thephotovoltaic package 607 comprises a back barrier 714, a device layer791, and a front barrier 711. In a representative embodiment, the frontbarrier 711 and the back barrier 714 can be a sheet or layers ofplastic, such as EVA and/or PET. The back barrier 714 can also include ametal such as a metal foil. The photovoltaic arrays 510, capacitor 615,the DC/DC converter 670, the printed circuit board 792, and theelectrical connections 793 are floating and sealed between the frontbarrier 711 and the back barrier 714 with an encapsulant 713.

Referring again to FIG. 6, the photovoltaic array 510 can include manyseries-connected thin-film photovoltaic cells. Thin film photovoltaicpower cells can be constructed by deposition of thin layers of materialssuch as amorphous silicon (a-Si), copper-indium-gallium diselenide(CIGS), etc. Each photovoltaic cell produces a low DC voltage, typicallya fraction of one volt. A manufacturing technology capable ofinexpensively connecting many of these cells in series is employed, suchas that described in U.S. patent application Ser. No. 11/451,616, titledPhotovoltaic Module With Integrated Current Collection AndInterconnection, filed Jun. 13, 2006, so that the photovoltaic array 510produces a relatively high voltage DC output at its peak power operatingpoint with rated solar irradiation. For example, when the utilityvoltage is 120 Vrms, the PV output voltage for a single photovoltaicelement module can be in the vicinity of several tens of volts. The PVoutput voltage of an photovoltaic array 510 may typically be in thevicinity of 20 VDC. The photovoltaic array 510 can include diodes(“backplane or bypass diodes”) that protect the photovoltaic array 510in the event that the photovoltaic array 510 is partially shadowed,shaded, or has irregular illumination as described in U.S. patentapplication Ser. No. 11/812,515, titled Photovoltaic Module Utilizing AnIntegrated Flex Circuit And Incorporating A Bypass Diode, filed Jun. 19,2007 which is herein included by reference. Backplane diodes affect thevoltage produced by the photovoltaic array under partially shadedconditions. Each diode is connected in an anti-parallel manner acrossone or more photovoltaic cells; the short-circuit current produced bythe photovoltaic array depends on a variety of factors including thesolar irradiation.

The energy storage element, such as a capacitor 615 comprises an energystorage element connected across the terminals of the photovoltaic array510 (i.e. the capacitor 615 is in series with the photovoltaic array510). The capacitor 615 keeps the instantaneous power flowing out of thephotovoltaic array 510 approximately constant and equal to the maximumpower that the photovoltaic array 510 is capable of producing. Hence,the capacitor 615 maximizes energy capture.

Conventional PV systems employ electrolytic capacitors for this purpose;however, electrolytic capacitors do not exhibit the very low profilerequired for integration into a low-profile module, nor do they meet therequirements of long life and high temperature operation. In arepresentative embodiment, the capacitor 615 can be a ceramic chipcapacitor. Ceramic chip capacitors exhibit low profiles of less than 11mm and are capable of high temperature operation. Ceramic capacitors canbe used in the photovoltaic power module 100 because the power levelsare so low in the photovoltaic power module 100 that the capacitancerequired is small. Hence, the total capacitance desired at theapplicable voltage rating is available in a ceramic capacitor.

The DC/DC converter 670 includes a transformerless DC/DC converter. Theterm transformerless means that the DC/DC converter power does not flowthrough a transformer. However, the device may contain a transformer forfunctions other than power processing, such as to couple a MOSFET gatedrive signal between the controller circuit and the MOSFET gate or usinga small transformer as current-sensing device to transmit a signalproportional to the transistor or diode current to the controller, etc.The DC/DC converter 670 is a low-profile and high-efficiency converterwhich enables its integration into a thin film module package. The DC/DCconverter 670 can be capable of producing an output voltage that is lessthan or greater than the input voltage. Hence, the DC/DC converter 670can be a buck converter with a pass-through path, a boost converter witha pass-through path, a buck converter followed by a boost converter(preferably with a pass-through path), or a buck-boost converter(preferably with a pass-through path). In a representative embodiment,the buck converter with a pass-through path, the boost converter with apass-through path, the buck converter followed by the boost converter(preferably with a pass-through path), or the buck-boost converter(preferably with a pass-through path) are transformerless. In theembodiment of FIG. 6, the DC/DC converter 670 includes a buck converter550 followed by a boost converter 560 or it can be a buck-boostconverter. In a representative embodiment, the buck converter followedby the boost converter, or the buck-boost converter are transformerless.Alternatively, any other device that can produce an output voltage thatis less than or greater than the input voltage can be used.

The DC/DC converter 670 can be synchronous or asynchronous. Anasynchronous buck converter, for example, can include a transistor, adiode, and an inductor. In asynchronous operation, the transistorswitches with a particular duty cycle that results in a lower voltage atthe output. A synchronous buck converter, for example, can include twotransistors and an inductor (i.e., the diode of the asynchronousconverter is replaced by a transistor, such as a MOSFET). In synchronousoperation, the two transistors switch alternately with a particular dutycycle that results in a lower voltage at the output, and the controlleris modified turn on the additional transistor when the first transistoris off, and optionally also to turn off the additional transistor whenthe inductor current passes through zero. Likewise, a synchronous orasynchronous boost converter, buck converter followed by a boostconverter, or buck-boost converter can be used as part of DC/DCconverter 670. Alternatively, in synchronous implementations, a diodecan be employed to allow current flow during short delays (dead times).

The DC/DC converter 670 optionally also includes a pass-through path.The pass-through path directly connects the photovoltaic arrays 510 tothe output of the DC/DC converter 670. The pass-through path can bedirectly integrated into the DC/DC converter 670 or parts of the DC/DCconverter 670 such as the buck converter 550 and the boost converter560. In a representative embodiment, the transistors of the buckconverter 550 and the boost converter 560 can stop switching and form apass-through path from the photovoltaic arrays 510 to the output of theDC/DC converter 670. Alternatively, a separate pass-through path orpaths can exclude the DC/DC converter 670.

The DC/DC converter 670 can operate in at least three modes: buck mode,boost mode, and pass through mode. In buck mode, the transistors of thebuck converter 550 switch and the transistors of the boost converter 560do not switch thereby reducing the voltage at the output of the DC/DCconverter 670. In boost mode, the transistors of the boost converter 560switch and the transistors of the buck converter 550 do not switchthereby increasing the voltage at the output of the DC/DC converter 670.In pass through mode, the transistors of the buck converter 550 and theboost converter 560 do not switch, and a pass-through path directlyconnects the photovoltaic arrays 510 to the output of the DC/DCconverter 670; preferably so that the DC/DC converter 670 passes thepower of the photovoltaic arrays 510 without changing the current orvoltage. The controller 680 decides which mode will cause the module toproduce the most power by testing each of the modes periodically. Forexample, the controller 680 determines the power produced in each of thebuck mode, boost mode, and pass through mode. The controller 680 thenoperates the module in the mode that produced the most power. Thecontroller 680 perturbs the module periodically to determine if a newmode should be selected.

To achieve a low profile of several millimeters or less, the DC/DCconverter must operate with a high switching frequency, typicallyseveral hundred kilohertz or more. However, a high switching frequencytypically leads to high switching loss, and hence low efficiency asnoted above with regard to the “shuffle” converter approach. However,the shuffle approach requires bidirectional converters and more complexinterconnections. Instead, the DC/DC converter 670 ceases switchingunder balanced conditions (when all photovoltaic arrays have the samemaximum power point), leading to very high efficiency under nominaloperating conditions. Switching, with the associated loss, occurs onlywhen substantial system imbalances exist. The DC/DC converter 670 isconnected directly in series with the photovoltaic array 510. This highefficiency and simple interconnection allows the converter to beintegrated directly into a photovoltaic package 607 with a low profile.

In addition, to achieve a low profile of several millimeters or less,while also meeting current waveform requirements such as IEEE Standard1547, the DC/DC converter 670 operates with a high switching frequency,typically 100 kHz or more. However, a high switching frequency typicallyleads to high switching loss, and hence low efficiency. The DC/DCconverter 670 can optionally employ the discontinuous conduction mode orthe boundary conduction mode to avoid these switching losses and achievehigh efficiency operation. In discontinuous conduction mode, theinductor current of an inductor of the DC/DC converter goes to zero forat least a period of time before the DC/DC converter cycles or switches.In boundary conduction mode, the inductor current of an inductor of theDC/DC converter goes to zero for an instant before the DC/DC convertercycles or switches. The buck portion and boost portion of the DC/DCconverter can be operating simultaneously and in different modes.

The optional EMI filter (not shown) separates the high-frequencyswitching elements of the DC/DC converter 670 and the inverter. Meetingregulatory limits on conducted EMI, such as those imposed by FCC Part 15Subpart B, requires that a filter be placed between the high-frequencyswitching elements and an AC utility. Conventional inverters employ ACEMI filters for this purpose, which typically include high-profileAC-rated capacitors. The EMI filter employs a DC EMI filter that useslow-profile DC-rated capacitors. This is achieved by positioning the EMIfilter on the DC side of the inverter 120 which is located outside ofthe photovoltaic element module 505, as shown in FIG. 5, and by avoidinghigh-frequency switching of inverter elements. The DC side of theinverter is the power input of the inverter. The AC side of the inverteris the power output of the inverter. The optional EMI filter can beintegrated in each photovoltaic element module 505 or located at theinverter 120. If the EMI filter is integrated into the module, then itcan be placed at the output of the boost converter, the input of thebuck converter, or both.

The photovoltaic element module 505 is controlled by a controller 680.The controller 680 provides the duty cycle modulation and/or frequencymodulation, required to drive the switching transistors of DC/DCconverter 670 and maintain operation in the discontinuous or boundaryconduction modes. Controller 680 performs the functions of maximum powertracking, pass-through perturbing, selecting appropriate transistor dutycycles to interface to the given string current, output voltagelimiting, and shutdown modes. In a representative embodiment, some orall of the control functions are realized through the use of digitalcircuitry, enabling a greater degree of sophistication. The controller680 can be a central, integrated controller or, alternatively,individual sections of the photovoltaic element module 505 can havededicated controllers. For example, the buck converter 550 and boostconverter 560 can have separate controllers. Likewise, the pass-throughcan be operated with a separate controller. Optionally, the controller680 can use voltage, current or other information from the photovoltaicarray 510, the energy storage device, such as a capacitor 615 or anotherstorage device, a DC/DC converter 670, the buck converter 550, or theboost converter 560.

When a plurality of photovoltaic element modules 505 are combinedtogether, the resulting system of “smart PV modules” is able to adapt toa changing environment, maximizing energy capture in the presence ofcomplex shadows, shading, and reflections. With the addition ofcommunications capability, it is further possible to obtain operationaland performance data on a fine scale.

Referring to FIG. 8, a circuit of the photovoltaic element module 505 ofFIG. 6 in accordance with a representative embodiment is shown. Thephotovoltaic element module 505 achieves high efficiency under nominalconditions by employing a converter architecture whose efficiency ismaximized under nominal balanced conditions, switching and convertingpower only as necessary to mitigate imbalances between photovoltaicmodules. In the photovoltaic element module 505, the DC/DC converter isrealized by the cascade connection of buck and boost converters.

The photovoltaic element module 505 includes an photovoltaic array 510,an energy storage device, such as a capacitor 615 or another storagedevice, a DC/DC converter 670, and a controller 680. The DC/DC converter670 can include a buck converter 550 and a boost converter 560. The buckconverter 550 includes transistor 851 (P₁), transistor 852 (N₁),optional diode 853 (D₁), and inductor 854 (L). The boost converter 560includes transistor 861 (P₂), transistor 862 (N₂), optional diode 863(D₂), and capacitor 864 (C₂). The two converter functions can becontrolled independently. Transistor 851 (P₁) and transistor 852 (N₁),together with inductor 854 (L), provide a buck converter function, andtransistor 862 (N₂) and transistor 861 (P₂), with inductor 854 (L)provide a boost converter function.

A string bypass diode 890 (D₃) is included to improve fault tolerance.If for some reason a DC/DC converter fails to operate (for example, ifit behaves as an open circuit), then diode D₃ allows a path forconduction of the string current I_(string), so that the remainingelements of the series string are able to deliver their power to theinverter.

Component values for a discrete circuit realization of the DC/DCconverter stage, having a full power photovoltaic array input of 20 V at1 A, a maximum string current of I_(string)=1 A, and a maximum outputvoltage of V=20 V, are, for example, as follows. Transistor N₁:n-channel power MOSFET rated 30 V, such as that available from ONSemiconductor as part number NTGS4141N. Transistor P₁: 30 V p-channelMOSFET, such as that available from ON Semiconductor as part numberNTGS4111P. Transistor N₂: 60 V n-channel MOSFET, such as that availablefrom ON Semiconductor as part number NTF3055-100. Transistor P₂: 60 Vp-channel MOSFET, such as that available from ON Semiconductor as partnumber NTF2955. Filter inductor L: 100 μH at 1.5 A, low profile.Capacitor C₁: two 25 V, 4.7 μF X7R multilayer ceramic capacitorsconnected in parallel, each such as that available from Murata as partnumber GRM31CR71E475KA88L. Capacitor C₂: two 50 V 2.2 μF X7R capacitorsconnected in parallel, each such as that available from Murata as partnumber GRM31CR71H225KA88L. Diode D₁: 30 V Schottky diode, such as thatavailable from ON Semiconductor as part number MBRO530T3G. Diode D₂: 60V Schottky diode, such as that available from ON Semiconductor as partnumber SS16. Diode D₃: 60 V Schottky diode, such as that available fromON Semiconductor as part number SS26.

Under nominal balanced conditions, in a nominal range, the maximum powerpoints of every photovoltaic element module 505 occur at the samecurrent. A string current 895 can therefore be chosen to be equal tothis optimal current, and the DC/DC converters can directly connecttheir photovoltaic arrays to the series string. The controller 680, oroptionally a pass-through controller, tests for nominal balancedconditions by momentarily operating the photovoltaic element module 505in each of a pass-through mode, a buck mode, and a boost mode. Thecontroller 680 chooses the mode that produces the most power. Hence, thenominal range can be defined as the conditions where the pass-throughmode produces more power than the buck mode or the boost mode.Alternatively, the nominal range is plus or minus ten percent of theoptimal string current required to maintain a constant voltage at theinput of the inverter. Here, the controller 680 chooses the pass-throughmode. When the pass-through mode produces the most power, the controller680, or optionally a pass-through controller, asserts a pass-throughmode by leaving transistor 851 (P₁) and transistor 861 (P₂) in the ONstate, and transistor 852 (N₁) and transistor 862 (N₂) in the OFF state,without high frequency switching. Hence, in pass-through mode, thephotovoltaic array is connected through transistor 851 (P₁), transistor861 (P₂), and inductor 854 (L) to the series string, and currentI_(string) (string current 895) flows through the photovoltaic array510. The transistor 851 (P₁) and transistor 861 (P₂) in the ON state,and transistor 852 (N₁) and transistor 862 (N₂) in the OFF statecomprise a pass-through. The pass-through comprises a low resistancepath from the photovoltaic array 510 to the output of the DC/DCconverter 670. As described above, the pass-through can be integratedinto the buck converter 550 and/or the boost converter 560.Alternatively, a separate pass-through path or paths can exclude thebuck converter 550 and/or the boost converter 560.

In the event that one of the photovoltaic array 510 is partially shaded,then the current at its maximum power point is reduced. To interfacethis reduced photovoltaic array current to the larger string currentI_(string), (string current 895) the DC/DC converter 670 must increasethe current. The controller 680, or optionally a pass-throughcontroller, tests for nominal balanced conditions by momentarilyoperating the photovoltaic element module 505 in each of a pass-throughmode, a buck mode, and a boost mode. Here, the controller 680 choosesthe buck mode. The controller 680 accomplishes this through highfrequency switching of transistor 851 (P₁) and transistor 852 (N₁).Transistor 851 (P₁) is turned ON by the controller 680 for a durationt₁, with transistor 852 (N₁) in the OFF state. At the end of thisinterval, the controller turns transistor 851 (P₁) OFF and turnstransistor 852 (N₁) ON, for a second interval of duration t₂. At theconclusion of the second interval, the process repeats. The switchingperiod T_(s) is defined as T_(s)=t₁+t₂. The duty cycle D₁ of transistor851 (P₁) is defined as D₁=t₁/T_(s). In this mode of operation (calledhere the “buck mode”), transistor 861 (P₂) and transistor 862 (N₂) arepreferably not switched at high frequency, with transistor 862 (N₂)remaining in the OFF state and transistor 861 (P₂) remaining in the ONstate. The ratio of the photovoltaic current I_(pv) of the photovoltaicarray 510 to the string current I_(string) (string current 895) is thengiven approximately by I_(pv)/I_(string)=D₁. Since the duty cycle mustlie in the range 0≦D₁≦1, the controller 680 is able to choose a dutycycle D₁ to cause the photovoltaic array 510 to produce its maximumpower when the maximum power point occurs at an array current I_(pv)that is less than the string current I_(string). The controller 680includes a maximum power tracking algorithm that, either directly orindirectly, causes transistor 851 (P₁) to operate substantially at thisduty cycle. In the buck mode, the high frequency switching of transistor851 (P₁) and transistor 852 (N₁) leads to additional power loss notpresent under nominal balanced conditions. Also in the buck mode, theDC/DC converter 670 reduces the voltage: the converter output voltage Vis given approximately by the duty cycle multiplied by the voltage ofthe photovoltaic array V=D₁ V_(pv). Hence the effect of partial shadingof a photovoltaic module is to reduce the output voltage of itscorresponding DC/DC converter V, and hence also to reduce the totalstring voltage V_(string).

To maximize the efficiency of the inverter (element 120 in FIG. 5) andreduce its cost, it is advantageous to minimize variations in the totalstring voltage V_(string). Hence, when shading of one photovoltaic arraycauses its DC/DC converter to reduce its output voltage V, the remainingconverters of the photovoltaic element modules of the string mustincrease their output voltages to maintain a constant total stringvoltage V_(string). This is accomplished through high frequencyswitching of transistors P₂ and N₂ in the other converters.

With respect to the remaining converters of the photovoltaic elementmodules, the respective controllers test for nominal balanced conditionsby momentarily operating the remaining photovoltaic element modules ineach of a pass-through mode, a buck mode, and a boost mode. Here, thecontrollers choose the boost mode. In the remaining photovoltaic elementmodules, the controller 680 turns transistor 862 (N₂) ON for an intervalof length t₁, with transistor 861 (P₂) in the OFF state. At the end ofthis interval, the controller 680 turns OFF transistor 862 (N₂), andturns ON transistor 861 (P₂). Transistor 861 (P₂) then conducts for asecond interval of length t₂. At the end of this second interval, theprocess repeats. The switching period is defined as T_(s)=t₁+t₂, and theduty cycle in this mode (called here the “boost mode”) is defined asD₂=t₁/T_(s). In the boost mode, the controller leaves transistor 851(P₁) always in the ON state, and transistor 852 (N₁) always in the OFFstate. The current ratio is now given approximately byI_(pv)/I_(string)=1/(1−D₂), and the voltage ratio is given approximatelyby V/V_(pv)=1/(1−D₂). Since 0≦D₂≦1, the DC/DC converter now increasesthe voltage, and the string current I_(string) is less than thephotovoltaic array current I_(pv). Since transistor 862 (N₂) andtransistor 861 (P₂) switch at high frequency in the boost mode,additional losses are incurred relative to operation under nominalbalanced conditions.

The DC/DC converter 670 optionally includes diode 853 (D₁) and diode 863(D₂) to assist during the high-frequency switching transitions. Shortdelays or “dead times” are introduced into the high-frequency switchingtransitions because, in order to achieve high efficiency, transistor 851(P₁) and transistor 852 (N₁), or transistor 862 (N₂) and transistor 861(P₂), must not simultaneously conduct. To ensure this, the controller680, between the turning off of one transistor and the turning on of thenext transistor, typically introduces short delays. During this deadtime diode 853 (D₁) or diode 863 (D₂) provide a path for conduction ofthe inductor current.

As noted previously, it is desirable that the total string voltageV_(string) of the photovoltaic element modules be maintained at asubstantially constant value, and that when some of the photovoltaicarrays experience shading, the DC/DC converters of the remainingphotovoltaic arrays increase their output voltages. This functionalitycould be attained either through a central controller that communicatesthe desired control actions to each DC/DC converter module, or by theactions of local autonomous controllers within the DC/DC convertermodules themselves. A preferred embodiment is described here, thatimplements the second approach. It is further desired that the maximumpower point algorithms of the individual photovoltaic element modules benon-interacting, so that a change in one photovoltaic element module ofthe series string does not disrupt the operating points of the otherphotovoltaic element modules in the string.

Referring to FIG. 9, a circuit of a controller of the photovoltaicelement module of FIG. 8 in accordance with a representative embodimentis shown. The photovoltaic element module 505 includes an photovoltaicarray 510, an energy storage device, such as a capacitor 615 or anotherstorage device, a DC/DC converter 670, and a controller 680. The DC/DCconverter 670 can include a buck converter 550 and a boost converter560. The buck converter 550 includes transistor 851 (P₁), transistor 852(N₁), optional diode 853 (D₁), and inductor 854 (L). The boost converter560 includes transistor 861 (P₂), transistor 862 (N₂), optional diode863 (D₂), and capacitor 864 (C₂). The two converter functions can becontrolled independently. A string bypass diode 890 (D₃) is included toimprove fault tolerance.

The controller 680 includes a maximum power point tracker 910, acompensator 920, and a driver module 930. The driver module 930 includesa mode selector, a pass-through controller, a pulse-width modulator,gate drivers, and a limiting and protection module. The pass-throughcontroller can sense when the string current is in the nominal range(for example, by testing the module in buck mode, boost mode, andpass-through mode) and place the photovoltaic element module intopass-through mode.

The controller 680 contains an inner feedback loop that adjusts thetransistor duty cycles such that the photovoltaic array voltage V_(pv)substantially follows a reference signal V_(ref). regardless ofvariations in the string voltage or string current. A maximum powerpoint tracking algorithm, implemented by the maximum power point tracker910, adjusts the value of V_(ref) such that the power produced by thephotovoltaic array P_(pv)=V_(pv)I_(pv) is maximized. The maximum powerpoint tracking algorithms can be, for example, a “perturb and observe”(P&O) algorithm. Because of the inner feedback loop, the maximum powerpoint tracker 910 need not adjust its output V_(ref) in response tovariations in the string current I_(string) or the converter outputvoltage V. Consequently, the maximum power point trackers of the variousphotovoltaic element modules of the series string are decoupled,eliminating system transient and stability problems caused byinteractions between multiple maximum power point tracking algorithms.

The controller 680 provides sufficient local autonomous control so thatthe photovoltaic element modules can operate efficiently withoutcommunication of control information between module blocks or betweenmodule blocks and a central controller. Hence, a system of photovoltaicelement modules can be regulated by merely adjusting the input current(i.e., I_(string)) of the inverter (element 120 in FIG. 5) such that thedesired voltage V_(string) is applied to the inverter input terminals.The reason for this is that the control 680 causes the converter outputterminals to exhibit a constant power source characteristic.

Referring to FIG. 10, a graph of a constant power source characteristicof an autonomous photovoltaic element module in accordance with arepresentative embodiment is shown. The power P_(pv) flowing into theconverter input terminals is independent of the output terminalquantities V and I_(string). Since the converter has a high efficiency,its equilibrium output power is also approximately equal to P_(pv).Therefore, the converter output terminals exhibit the equilibriumconstant power output characteristic VI_(string)=P_(pv) as illustratedby the current-voltage plot of a constant power source characteristicprofile 1010. Since all of the photovoltaic element modules of FIG. 5have their outputs connected in series and share the same currentI_(string), the module output voltages depend directly on theirrespective photovoltaic array powers. The total string voltageV_(string) is equal to the sum of the module output powers divided bythe string current I_(string). Hence the inverter can adjust its DCinput voltage V_(string) through control of the current I_(string) thatit draws at its input port. The constant power characteristic requiresthat the output quantities be limited under open-circuit andshort-circuit conditions; converter output voltage limiting and possiblyalso output current limiting modes are therefore required.

FIG. 11 is a graph of predicted efficiency of DC/DC converter powerstages under various operating conditions in accordance with arepresentative embodiment. A system of ten 20 W photovoltaic elementmodules connected in series string is analyzed. Under nominal balancedoperating conditions (where each module produces the same power, equalto the rated power), each photovoltaic element module produces 20 V at 1A, for a total string output of 200 V at 1 A. Profile 1130 shows thestring current (A) versus the efficiency where all photovoltaic elementmodules receive 1000 W/m². Profile 1120 shows the string current (A)versus the efficiency where all photovoltaic element modules receive 500W/m². Profile 1110 shows the string current (A) versus the efficiencywhere all photovoltaic element modules receive 200 W/m². Profile 1140shows the string current (A) versus the efficiency of a 0 W/m² modulewhere the remaining nine photovoltaic element modules receive 1000 W/m².It can be seen that the predicted efficiencies under balanced conditions(where each module produces the same power) are very high, not only atfull power (solar irradiance of 1000 W/m²), but also at half power (500W/m²) and at 20% power (200 W/m²). The analysis assumes that the centralinverter adjusts the string current as necessary to maintain a totalstring voltage of V_(string)=200 V. Unbalanced conditions lead todegradation of the efficiency of the lower power modules. Profile 1140illustrates the effect of partial shading of one module, while the othernine modules operate with full solar irradiance. Profile 1140 is thelocus of the efficiency of the partially shaded module. At a solarirradiance of 200 W/m², the efficiency of the converter serving thepartially shaded photovoltaic array has decreased by approximately 10%,because of the resulting large voltage step-down ratio.

Referring to FIG. 12, a graph of a simulation of a turn-on transient ofa ten photovoltaic element module system in accordance with arepresentative embodiment is shown. The turn-on transients of a tenphotovoltaic element module system were simulated using MATLAB/Simulink.The ten photovoltaic element module system is a 200 W photovoltaicelement module string. The ten photovoltaic element modules in seriesdrive the input port of an inverter. Seven of the photovoltaic elementmodules operate with a nominal solar irradiance of 1000 W/m². Two of thephotovoltaic element modules operate under partially shaded conditions,with solar irradiances of 400 W/m² and 200 W/m². The remainingphotovoltaic element module operates with a solar irradiance of 1200W/m². The responses of the non-interacting perturb-and-observe maximumpower point tracking algorithms are illustrated by response ofphotovoltaic element modules 1-10 (1201-1210). The response ofphotovoltaic element modules 1-10 (1201-1210) shows the respectivemodule output voltage (V) versus time (t) for about 1 second. Responses1201-1207 represent the 1000 W/m² cells. Response 1208 represents the200 W/m² cell. Response 1209 represents the 400 W/m² cell. Response 1210represents the 1200 W/m² cell. These waveforms are plots of the voltagesV_(ref) commanded by the maximum power point trackers of each moduleversus time. The photovoltaic array maximum power points occur atvoltages in the vicinity of 20 V. The perturb-and-observe algorithms arewell behaved, and reach their equilibrium operating points after severaltenths of a second. Typical P&O step sizes are 10 mV, and step times arein the vicinity of 2-5 msec. The module output voltages are distributedin proportion to the output powers.

The foregoing description of the exemplary embodiments have beenpresented for purposes of illustration and of description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Forexample, the described exemplary embodiments focused on anrepresentative implementation of a buck-boost converter forimplementation on a 120V AC utility grid. The present invention,however, is not limited to a representative implementation as describedand depicted. Those skilled in the art will recognize that the deviceand methods of the present invention may be practiced using variouscombinations of components. Additionally, the device and method may beadapted for different utility grid standards. The embodiments werechosen and described in order to explain the principles of the inventionand as practical applications of the invention to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications as suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents. U.S. patent application Ser. No.______ (Attorney Docket Number 075122/0146), titled Thin-FilmPhotovoltaic Power System With Integrated Low-Profile High-EfficiencyInverter, filed on Feb. 13, 2009 is herein incorporated by reference inits entirety.

What is claimed is:
 1. A photovoltaic device, comprising: at least onephotovoltaic cell; and a DC/DC converter electrically coupled to the atleast one photovoltaic cell, wherein the at least one photovoltaic celland the DC/DC converter are integrated into a photovoltaic package. 2.The device of claim 1, wherein the DC/DC converter comprises atransformerless buck converter and a transformerless boost converter ora transformerless buck-boost converter.